Since the late 1950s, dental crowns, bridges, and the like have been made with a composite including a cast metal substrate with a veneer of porcelain fabricated in such a manner that there is a bond between metal and porcelain such that the composite is stronger than the individual component parts.
There are several aspects to be addressed when formulating such composites. Aesthetics is one aspect to be considered. The primary reason for the use of such a composite is to reproduce the normal coloration of natural dentition. The enamel layer of healthy natural dentition is quite translucent and porcelain can be made with equal translucency. The translucency of enamel allows the color of healthy dentine to be seen. The dentine color normally has a yellowish tint. For a porcelain/alloy combination to be effective as a composite, a layer of oxide must be present on the alloy to form a bond with the porcelain. While high gold alloys may provide a suitable yellowish background for the porcelain for proper aesthetics, the alloying elements can form a dark gray to black colored oxide layer, which can screen out this underlying yellowish background color. Moreover, larger amounts of alloying elements form a colored oxide layer that can further reduce or eliminate the underlying gold color of the alloy.
Mechanical properties is another aspect to be considered. The American National Standards Institute/American Dental Association (“ANSI/ADA”) specification #38 and International Organization for Standardization (“ISO”) standard IS9693 require a yield strength of at least 250 megapascal (“MPa”) for the alloy. To attain such strength in gold-based alloys, significant amounts of alloying elements must be added, the result being alloys of “yellow” color that are nearer to gray. It was thought necessary to provide great strength because the alloy supported porcelain, which had little strength, particularly in tension, and zero ductility. Any slight deformation of the metal can cause fracture of the porcelain layer. The minima for the standards mentioned were set on the basis of testing alloys that were being successfully used at the time of the development of the standards. Subsequently, the minimum requirement has been questioned since alloys with less than this minimum have been used successfully. Also, it has been shown that the minimum requirement for single crowns should be lower than that for crowns composed of three or more unit bridges.
An unpublished work at the University of Kiel in Germany has indicated that from 30 to 35 kilograms of force causes pain to patients while, in one instance, 75 kilograms of force caused fracture of the tooth.
Physical properties is another aspect to be considered. Although the above-mentioned standards do not require either minimum or maximum values for the coefficient of thermal expansion (“CTE”), these standards require that the CTE value be given for both porcelain and alloy. This is because the popular conception is that the coefficients of porcelain and metal should be “matched” in order to assure compatibility of the two. This concept fails to take into consideration that stresses between the two occur during cooling rather than during heating and the cooling rates of porcelain and metal vary very significantly.
It is readily understood that the solidus of the alloy must be sufficiently higher than the firing temperature of the porcelain so that the alloy is not even partially melted during firing of the porcelain.
Chemical properties is another aspect to be considered. The bonding of porcelain to metal does not occur directly between porcelain and metal, rather it occurs between porcelain and the metal oxide layer. Normal PFM procedure is to heat the cast alloy to a suitable temperature to produce a metal oxide layer on the surface of the alloy. If this oxide is not adherent to the alloy; it can be simply removed by its attachment to the porcelain. Some of the bond is simply mechanical but the primary bonding takes place as a mutual solution of metal oxide in porcelain and vice versa. If the oxide is not soluble in the porcelain and/or vice versa, no bonding takes place. When the porcelain is fired, small particles and larger particle surfaces are fused (melted) and this liquid porcelain and the metal oxide layer form a solution by either liquid or solid diffusion.